Abstract:

To investigate the soil cadmium（Cd）pollution of Hunan citrus production areas，and reveal the effects of soil Cd pollution on the development of citrus plant，this study aims to provide theoretical basis for the prevention and control of Cd pollution in citrus production areas. Firstly，the soil pH，organic matters and Cd content in the main citrus producing areas of Hunan Province and Changsha-Zhuzhou- Xiangtan（Changzhutan）area were determined. Meanwhile，the Cd content in fruits of‘Juxiangzao’ Satsuma mandarin（Citrus unshiu Marc）and‘Newhall’navel orange（C. sinensis Osbeck）were also evaluated. And then，three-year-old‘Juxiangzao’Satsuma mandarin and‘Newhall’ navel orange trees were treated with different concentration of CdCl₂ · 2H₂O（1.0，1.5，3.0 and 10.0 mg · kg^-1）after which the physiological and biochemical changes of plants under Cd stress were analyzed. Afterward，the citrus rootstocks，trifoliate orange（Poncirus trifoliata）and Carrizo citrange（C. sinensis × P. trifoliata），were subjected to Cd stress by using 50 mg · L^-1 CdCl₂ · 2H₂O and the Cd tolerance of these two rootstocks were observed. According to the 278 soil samples in which 201 samples were from the main citrus producing areas and 77 samples were from Changzhutan area，soil acidification（pH < 5.5）occurred in 70.5% of citrus orchards，and 48.56% orchards were with optimum soil organic matter（20.0 to 30.0 g · kg^-1）. Among them，the Cd contents in 93.03% orchard soils from the main citrus producing area were lower than the environmental soil quality standard（< 0.3 mg · kg^-1），but Cd contents in 71.43% orchard soils from Changzhutan area exceeded the standard. In the 55 orchards with Cd polluted soil，the Cd contents in the edible part of citrus fruits（43 Satsuma mandarin and 12‘Newhall’navel orange）were all lower than the national food safety standard（< 0.05 mg · kg^-1）. With the Cd treatments for‘Juxiangzao’ Satsuma mandarin and‘Newhall’navel orange trees，the root neck thickness，plant height and chlorophyll showed no significant difference with the control，while the fruit size was significantly impaired relative to the control. Cadmium accumulation varied greatly among plant organs，with a sequential decrease in root，stem，leaf and pulp. Further，the damage of soil Cd pollution to citrus plant was detected after long-term stress. After being subjected to cadmium stress for 180 days，the leaf MDA，Pro，and POD activities increased significantly. After Cd stress treatment（50 mg · L^-1 CdCl₂ · 2H₂O），the leaves of trifoliate orange were reddish and detached，while that of Carrizo citrange showed no significant difference to the control. The results above indicate that most of citrus orchards in the main citrus producing areas of Hunan Province are not polluted by Cd，but the serious pollution is found in Changzhutan area. Citrus fruits produced from the orchard with 0.30 to 7.46 mg · kg^-1 soil Cd meet the national standard for food safety. Soil Cd is mainly accumulated in the root of citrus trees，and the tree exhibits the weak ability of Cd transportation to the above-ground. Under Cd stress，Carrizo citrange has better Cd tolerance relative to trifoliate orange.

Key words: citrus, soil, heavy metal, Cd, development